I just had a tune-up, just the wires and plugs. Replaced the water pump and timing belt. It cost about $1200 at a local shop. When I drive on the belt-way the car starts miss firing around 70mph. When I let off the gas it seems to go away until I press the gas again. It comes and goes when it wants to,the car will be driving fine and then it starts again. Today my check engine light and trac light came on. Took the car out a little while ago and the light come on and off. Also while idling at a light I feel a small jump in the car like somethings wrong with the rpm. I really need help.

 

What type of sparkplugs did you use? Check if the spark plugs are gapped right. Check the spark plug wires if they are plug in the right port. Heres an illustration I found on search.


It could also be vaccum leak, injector problem, fuel filter, air filter, timing?

Dude update your profile. What year is your LS? Check your ECU for codes? And lastly, did you call that shop that did the tune-up? For $1200, they better finish the job right.

 

I would like to know the same thing. The 90 LS400 I am considering to buy also has an occasional miss at idle. It has a little "jump" to it while at the traffic lights. I did not notice it at all while driving on the road.
I think the plugs and wires have already been changed. Thanks for any information.

 

I'd take it right back to the shop that did the work. They probably pulled a sensor when they were replacing the plugs/wires. Or disconnected a vacuum hose. If it wasn't doing this before you took it to the shop, it's something THEY DID while performing the tuneup. It doesn't sound too major. I've done this by accident in tight engine bays. Take a look in there and see if anything is hanging loose that looks like it shouldn't.